<html>
  <script>
    var gotHide = false;
    var gotOrientationChange = false;
    var attachedListeners = false;
    async function doRequestFullscreen() {
        return document.documentElement.requestFullscreen();
    }
    async function toggleFullscreen() {
      if (!attachedListeners) {
        attachedListeners = true;
        document.addEventListener("visibilitychange", function() {
          if (document.visibilityState !== 'visible') {
            gotHide = true;
          }
        });
        window.addEventListener("orientationchange", function() {
          gotOrientationChange = true;
        });
      }
      if (!document.fullscreenElement) {
          await doRequestFullscreen();
          await screen.orientation.lock("landscape");
      } else {
        document.exitFullscreen();
      }
    }
  </script>
  <body style="height:5000px" onclick="toggleFullscreen();">
    <p>A (mostly) empty page.</p>
  </body>
</html>
